Hydrogen embrittlement of a copper precipitation strengthened and niobium microalloyed HSLA-100 steel on cathodic changing in synthetic seawater has been studied using slow strain rate technique. The effects of potential applied for hydrogen changing, pre-charging with hydrogen and changes in strain rate have been studied. A loss in ductility in terms of drop in percent elongation and percent reduction in area has been observed, the effect being prominent at potentials beyond –900 mV (SCE), SEM fractography shows an increase in brittle quasi-cleavage features with decreasing potential. A hardening effect on hydrogen charging up to –700mV (SCE), followed by a softening effect, has been observed. Precharging has led to a similar behaviour, but an overall increase in the strength values compared to material without precharging.